In the prior art, burner blocks have commonly been made of refractory ceramics. This is a type of ceramic which has a long-standing use in the art for providing insulation, and as such is often used to form the wall of large furnaces or other structures. For the purposes of this invention, refractory materials are defined as materials having a melting point of at least 500 C. and a thermal conductivity of not more than 5 watts per meter per degree Kelvin (hereinafter abbreviated as W/(m K) at at least one temperature in the range from 15-1500 C. If there is a temperature within the range from 15-1500 C. at which a refractory has a modulus of rupture of not more than 30 megapascals (MPa), it is further designated herein as a "weak" refractory.
Although refractories are good insulators against heat, they do not usually have good resistance to thermal shock from rapid changes in temperature. Weak refractories have even less resistance to thermal shock than stronger ones. Resistance to thermal shock is a very complex phenomenon, which has been recently reviewed by P. Boch, "Thermal Shock Properties of Ceramics", Fracture of Non-Metallic Materials (1987). As described in this reference and the work reviewed in it, at least two different major theories and four different thermal shock parameters have been proposed for characterizing materials, and a wide variety of specific geometries have been theoretically studied. Resort to experiment is still necessary to determine adequate thermal shock resistance in most practical case, however.
If a burner block has insufficient thermal shock resistance for its service conditions, it will crack, often quite soon after being put into service. Once the block has cracked, it may fail to provide effective separation of the combustion zone from other spaces, proper definition of the shape of the combustion zone, and/or adequate insulation of the environment from the heat developed in a functioning combustion zone. Furthermore, any crack induces additional sources of mechanical stress, so that once initiated, cracking usually rapidly worsens if a cracked burner block is kept in service.